Zero-order diffractive filters (ZOFs), sometimes dubbed resonant gratings or guided mode resonant filters, are optical filters that are based on the resonant reflection of a leaky waveguide. Illuminated for example with non-polarized, polychromatic light, ZOFs can show characteristic color effects upon rotation and are therefore clearly identifiable.
ZOFs known in the art employ a waveguiding layer that has a diffractive microstructure defining at least one of its waveguiding boundaries. The diffractive microstructures, or otherwise stated, the diffractive gratings are isotropic, parallel or crossed. They possess a period Λ and a depth t. The period Λ is in most cases smaller than the wavelength of light for which ZOF is designed.
The waveguiding layer has a thickness c and is made of a material having an index of refraction nhigh that is higher than index of refraction (nlow) of the matter surrounding the waveguiding layer. The waveguiding layer is thus sometimes referred to as high-index waveguiding material or layer, and the matter surrounding the high-index waveguiding layer is sometimes referred to as low-index matter or layer. The low-index matter can have different indices of refraction. For example, a first low-index matter can be a solid substrate, whilst a second low-index matter can be ambient air, as outlined herein below in greater detail with reference to FIG. 2.
In order to obtain zero-order diffraction color effects that are recognizable by the human eye, a number of parameters have to be adjusted including grating period Λ, grating depth t, thickness c of the waveguiding layer, fill factor or duty cycle f.f.=p/Λ, grating profile or shape (rectangular, sinusoidal (cf. FIG. 1A), triangular or more complex) and the indices of refraction nhigh and nlow. The diffractive grating can be exposed to ambient air (cf. FIG. 1B).
Referring now to FIG. 2, a ZOF 200 as known in the art comprises in the x/y-plane a waveguiding layer 220 having lower boundary 222 engaging with a substrate 210. Upper boundary 221 of waveguiding layer 220 is formed as a diffractive grating 225, which includes a plurality of protrusions 227 that are spaced apart from one another, and which is at the interface between waveguiding layer 220 and ambient air 230. The physical properties of diffractive grating 225 are at least defined by its physical dimensions, and more specifically, by its grating period Λ, the width p of protrusions 227 and thickness c of waveguiding layer 220. The fill factor (ff) or duty cycle of diffractive grating 225, can be defined as ff=p/A and may be at least approximately equal 0.5 or 50%. Waveguiding layer 220 has an index of refraction that is higher than that of air and that of substrate 210.
Thusly configured, at least some of polarized or unpolarized polychromatic visible light 250 incident on diffractive grating 225 at an illumination angle Θin is coupled in waveguiding layer 220. Specifically, ZOF 200 may enable the resonant coupling of light of several diffraction orders and thus of several wavelengths into waveguiding layer 220. The diffraction orders and the wavelengths that will be coupled into waveguiding layer 220 depend on at least one parameter of diffractive grating 225, the thickness c of the waveguiding layer 220 and differences in the index of refraction between waveguiding layer 220, substrate 210 and ambient air 230.
In FIG. 2, the resonant coupling of incident light 250 into waveguiding layer 220 is schematically shown and exemplified as “+1” order light 253 and “−1” order light 254 having orders +1 and −1, respectively. Due to the higher index of refraction of waveguiding layer 220 compared to the one of ambient air 230 and substrate 210, “+1”-order light 253 and “−1”-order light 254 are totally internally reflected from upper boundary 221 and lower boundary 222 of waveguiding layer 220. However, a first portion of the zeroth-order (hereinafter: first zeroth-order light) 251 of incident light 250 is directly transmitted through waveguiding layer 220 and leaves ZOF 200 by propagation through substrate 210. In addition, a second portion of the zeroth-order (hereinafter: second zeroth-order light) 252 of light 250 is diffracted together with “−1” order light 254 into waveguiding layer 220. Both second zeroth-order light 252 and “−1”-order light 254 propagate in waveguiding layer 220 in opposite directions. After propagating over distance d in waveguiding layer 220, second zeroth-order light 252 is coupled out via diffractive grating 225.
“+1” order light 253 and “−1” order light 254 may continue propagating in waveguiding layer 220. In contrary to what is true for diffraction orders that are higher than zero, the angle Θout (which is defined with respect to the normal N of waveguiding layer 220) of the outcoupled second zeroth-order light 252 is equal to Θin.
The resonance condition for the outcoupling of first zeroth-order light 251 and second zeroth-order light 252, can be tailored for a certain wavelength or wavelength range for the outcoupled light. For example, the wavelength(s) of second zeroth-order light 252 outcoupled via diffraction grating 225 depends both on the viewing angle Θout and the rotational orientation φ of diffractive grating 225 with respect to a viewer 260. For each pair of angles φ and Θout a particular spectral range or color is reflected or transmitted.
The spectral characteristics of such ZOFs are therefore tuneable. The reflection spectra Rzero-order or transmission spectra Tzero-order are the most prominent examples of the spectral characteristics of ZOFs.
As long as the materials employed in a ZOF possess no substantial absorption, the transmission spectra are the complement of those in reflection.
More details concerning zero-order diffractive filters can be found in M. T. Gale, “Zero-Order Grating Microstructures” in R. L. van Renesse, Optical Document Security, 2nd Ed., pp. 267-287.
Due to the above-outlined color effects which are characteristic for ZOFs, they can be employed in conjunction with a variety of applications such as security-related, sensor-related, and pigment-related applications. Security-related applications include the employment of ZOFs in anti-counterfeiting or forgery protection of, for example, documents (e.g., passports, visas, government forms); products (e.g., pharmaceuticals); and payment instruments (e.g., banknotes, credit cards, tickets and cheques); smart cards; and the like.
Document U.S. Pat. No. 4,484,797 teaches a variable index-of-refraction optical medium of certain minimum thickness and periodicity with respect to the wavelength of incident light-if it meets certain specified constraints with respect to (1) relative indices-of-refraction of both its internal structure and that of its surroundings and (2) relative values of incident wavelength to periodicity and the relative indices-of-refraction-operates to produce both angularly-dependent subtractive-color filter reflection spectra and subtractive-color filter transmission spectra in accordance with its physical parameters. The methods for manufacturing the device taught in are based on vacuum deposition steps. Specifically, the medium is manufactured as laminated foil in roll-to-roll processes with thermally evaporated ZnS as the waveguiding layer deposited on foil substrates which were micro-structured by hot-embossing.
Document WO2006038120 teaches a security device comprising first zero order diffractive microstructure on a substrate, a second zero order diffractive microstructure, and an intermediate light transmissive layer, separating the two diffractive microstructures. The spacing between the first and second diffractive microstructures is small enough so that optical interferences are produced between the diffractive microstructures. A further light transmissive layer covers the second diffractive microstructure.
Document US2008024866 teaches ZOFs comprising a first layer having periodic diffractive microstructures and a second layer, wherein said first layer has a refractive index higher than said second layer by at least 0.2, and nanoparticles located in at least one of said layers which affect the refractive index of said at least one of said layers. The present invention further relates to methods of manufacturing such ZOFs, to the use such ZOFs e.g. in security devices and to the use of specific materials for manufacturing ZOFs. The manufacturing method disclosed is based on water based deposition of a porous layer on a foil substrate in a roll-to-roll process followed by a water based deposition of a polymeric waveguide layer and a subsequent microstructure embossing of the water soluble polymeric waveguide layer
Document WO2004/077468 teaches a grid structure used for protecting valuable articles. The inventive structure consist of at least a first part provided with a grid constant which is less than a wavelength at which said part is observable and embodied in the form of a relief structure whose relief height is defined in such a way that the zero-order grid image can be observed in a determined spectral range. Said part has a size less than 0.5 mm at least in one direction. For the manufacturing of the disclosed high-index refracting waveguide, vacuum-based deposition is disclosed.
US2003017580 teaches a manufacturing method of a ZOF employable as a biosensor. Specifically, US2003017580 teaches a method for fabrication of a calorimetric resonant reflection biosensor structure comprising: (a) applying a liquid or semi-solid material that is capable of being transformed or cured into a flexible solid over a rigid master structure; (b) transforming the liquid or semi-solid material into a flexible master structure, wherein the flexible master structure has the rigid master structure embossed into a first surface of the flexible master structure; (c) peeling the flexible master structure from the rigid master structure; (d) placing the first surface of the flexible master structure onto a liquid or semi-solid layer, wherein the liquid or semi-solid layer is on a rigid substrate; (e) transforming or curing the liquid or semi-solid layer into a solid layer; (f) peeling the flexible master structure from the solid layer; and (g) applying a high refractive index dielectric film or reflective material over the solid layer, whereby a calorimetric resonant reflection biosensor structure is fabricated.
Document US2007285782 teaches one or more zero-order diffractive pigments (ZOP) having both a particle distribution matrix material, and a layer of material in or on such a matrix material and having an index of refraction higher than that of the matrix material, and having a diffractive grating structure with a period in the range of 100 to 600 nm, which is smaller than the wavelength of light reflectable thereby in the zeroth reflection order. In such ZOPs the index of refraction of the matrix material is usually at least 0.25 less than that of the material of the layer, and the layer is typically of a thickness between 30 and 500 nm.